This invention relates to optical circulators that will be indispensable in optical communications.
Optical communication has been widely acknowledged as broader frequency band and bigger capacity communications with bright promise of future success, since it uses laser lights of one hundred thousand times higher frequencies than those of microwaves, and recent developements in low-loss optical glass fiber transmission lines (GFT lines) have marked an important step toward the optical communication, stimulating many researchers to prepare tentative devices for the optical communication. These devices are, however, not yet competitive with any in microwaves from the view point of technical maturity. Modulation and demodulation techniques for the optical communication are far below a satisfactory level and relevant circuit elements of various kinds are now under developement. The object of this invention is to provide a useful optical circulator for the optical communication.
An optical circulator as a nonreciprocal element acts circulation, which is explained as such that if incident signal waves are fed to three-port junction in the counterclockwise rotational turn, the signal waves emanate from the respective ports in the same rotational turn, as so does the microwave circulator. The optical circulator plays an essential role in two users' communication. For instance, if one communicates in the distance using a common transmission line for both sending and receiving their messages, it is necessary to select the desired signals carrying the partner's messages from two oppositely traveling signals at either end of transmitter or receiver. Selection of the desired signal is made using an optical circulator. Reduction of reflected signals from incomplete connections of GFT lines and other various discontinuities existing in GFT line, muliplex transmission of light signals and uses in research and development of various optical circuit elements are other instances in the circulator applications.
An optical circulator of the invention is set up using magneto-optic material (MO material). The MO material is characterized by optical anisotropy which causes magneto-optic dichronism, Faraday effect, and Kerr effect under biasing magnetic field. These phenomena are closely related with particular dispositions of the directions for the light traveling and magnetization of the MO material. In the disclosure of the invention, only the Faraday location that the light traveling direction is parallel to the magnetization vector or the direction of biasing magnetic field is appreciated. An example of MO material is aluminum or rare earth substituted YIG single crystal that may take large Faraday rotating angle and low loss factor in the ranges of more than 1.1 microns in near infrared region which covers the low-loss range of GFT lines.
It is important to say that an optical circulator can not be constructed after the analogy of microwave circulators, because of the extremely short wavelength of the laser light in contrast with the centimeter wavelengths of the microwaves and different physical properties of the MO material from those of ferromagnetic material. We can explain more explicitely about an optical circulator for the communication by use of the semiconductor laser beam at the wavelength of 1.3 micrometers. If one tries to apply the design principle of stripline or waveguide Y-junction circulator in the microwaves to the optical circulator of the present concern, it is necessary to construct its MO structure even more precisely to the degree of about one tenth of the wavelength, that is, to the 0.1 micrometer precision. Such high precision technique in machining, however, is practically difficult from the nowaday's skilled level. In addition, there are technical difficulties encountered practically. GFT lines for guiding laser beams are coupled with the MO structure, when mode conversion between GFT line modes and resonant modes of the MO structure, impedance matching between the coupled GFT lines and MO structure, and relevant circulation adjustments are the problem to be solved. In fact, a single mode GFT line has the diameter of a few microns for the core region, and a multiple mode GFT line the diameter of about fifty microns. So when these GFT lines are connected to as small a dimension of the MO structure as about one tenth microns, in handling mode conversions, impedance matching, and circulation adjustments extremely precise instruments, since the MO structure is even less than one tenth order in dimension in comparison with the single mode GFT lines. As is also known, the laser light beam has so highly concentrated power density that it may probably cause thermal heating and eventually, nonlinear effect will be induced. It is therefore inadmissible to extend the microwave circuit theory, as it is, to the optical region of interest.
The circulator embodiment of the invention bases upon newly designed configurations and performance mechanism which thoroughly differ from the prior arts of microwave circulators. One of the principal ideas in the invention is that an incident light will get sufficient Faraday rotation which is cumulated repeatedly on passages through an MO material in the direction parallel to the magnetization, as both ends of the MO structure are terminated to give rise to multiple reflections. To couple three GFT lines with the MO structure, a cubic prism is used as a signal coupler for three ports, each port having a transparent window at the Brewster angle for transmitting a signal light.
An optical circulator embodiment of the invention utilizes an MO lens system comprising MO lens elements to focus the signal light to make an image on an outlet window, and also to converge the light energy so effectively as to excite multiple resonant modes in the MO lens system and the circulator will enable the resonant modes or their coupled operating modes to perform such multiple circulation frequency operation, like diplexer operation and broadband operation, as have been achieved in microwave circulators. More detailed explanations of the invenion will be made below, referring to the drawings.